This invention relates to a tapered roller bearing, and more particularly to a tapered roller bearing in which a torque loss is reduced and further relates to a final reduction gear adapted to transmit driving torque from a driving power source, such as an engine to an axle, and more particularly to a final reduction gear in which a roller bearing has been improved.
A tapered roller bearing having an outer race, an inner race and a plurality of tapered rollers arranged between these races is compact, and capable of supporting a large radial load and an axial load and, moreover, being used at a high rotational speed. However, as compared with a ball bearing, a torque loss in slide contact portions of a larger end surface of the tapered roller and a larger collar (rib) surface of the inner race both of which exert an axial load on each other is large. Therefore, the reduction of torque becomes a problem from the viewpoint of the saving of energy.
As a tapered roller bearing 21 in which a torque loss due to the slide contact of the tapered roller with the inner raceway is reduced, such a tapered roller bearing has been proposed that includes an outer race 22, an inner race 23 and a tapered roller 24 as shown in FIGS. 8A and 8B, in which a larger end surface 25 of the tapered roller 24 is formed frusto-conically with a larger collar surface 26 of the inner race 23 formed as an inclined surface opposed to the larger end surface 25, (Patent Document 1).
Patent Document 1
JP-A-61-139310U
In the tapered roller bearing 21 disclosed in the Patent Document 1, the surface of the inner race 23 other than the surface portion thereof corresponding to an annular groove 27 provided therein contacts the tapered roller linearly so that the inner race has stress distribution shown by hatches of broken lines in FIG. 8A. When an axial load is exerted on the tapered roller, the tapered roller 24 is moved as shown in FIG. 8B toward a right upper portion of the drawing so as to run on the larger collar surface 26 of the inner race 23, and the inner race comes to have stress distribution shown by hatches of broken lines in the same drawing. Namely, in two portions, i.e. a portion between a smaller end surface-side end portion 24a of the tapered roller 24 and a smaller collar end portion 23a of a raceway surface of the inner race 23, and a portion between a body side end portion of the larger end surface 25 of the roller and the larger collar surface 26 of the inner race 23, specially large power comes to be exerted (occurrence of an edge load) on each other, so that an early seizure and the separation of the roller come to occur in the bearing 21. As a result, the function of the bearing is not displayed sufficiently. Moreover, the regulation of a pre-load and a clearance is difficult.
As a differential gear unit for a final reduction gear, a differential gear unit is used which is provided with a pinion shaft supported rotatably on a differential carrier and mounted at one end portion thereof with a pinion gear, a ring gear meshed with the pinion gear, a differential case fixed to the ring gear, and a pair of roller bearings supporting the differential case rotatably on the differential carrier.
The roller bearings in the differential gear unit have heretofore been made of tapered roller bearings so as to obtain a high rigidity thereof but the reduction of the torque of the bearings has become a problem from the viewpoint of improving the fuel consumption. Under the circumstances, using ball bearings the rotary torque of which is smaller than that of the tapered roller bearings has been proposed. (Refer to Patent Document 2.)
The Patent Document 2 discloses a final gear unit as well which is provided with a pinion shaft mounted at one end portion thereof with a pinion gear and adapted to receive driving torque transmitted from an engine thereto, and a pair of tapered roller bearings supporting the pinion shaft rotatably on the differential carrier. The related art tapered roller bearings are used as they are as the tapered roller bearings in this final gear unit.
Patent Document 2
JP-A-2000-161466
In a differential gear unit using ball bearings, the rigidity thereof usually lowers as compared with a differential gear unit using tapered roller bearings. Therefore, new problems arise which include an increase in the manufacturing cost occurring due to a change in materials and the like made so that low torque loss and a high rigidity of the differential gear unit stand together, and a fear of a decrease in the lifetime of the differential gear unit which will be caused by the use of the gear unit in an extraneous oil. The development of a differential gear unit solving these new problems and having bearings securing the low torque loss and high rigidity mentioned above is demanded. The final gear unit also has a problem of reducing the torque loss of the tapered roller bearings for the same reasons mentioned above.